The purpose of the applications is to build a set of forms
composed of user input entries and calculated outputs. The volume
of data managed is relatively high and its definition can suffer
changes periodically. So, a solution could be an application that
builds these forms using gtk or any other toolkit based on
definitions declared in a XML file.

libglade and some mapping logic may do what you need here.

Using gnumeric I can make use of its calc engine and apply it to
the output entries, and even its format capabilities for numbers.
The tool that interpretes the definition file should bind entries
using gnumeric functions. This way I get a flexible and easy tu
build app.
Is gnumeric suitable for this purpose ? What headers should I
study ? Any recommendation to begin hacking gnumeric ?

It may be a reasonable fit, but I still don't have a clear vision of
what you're try to do. Depending on your needs the best place in
the source to start poking around is to get it building. Get your
self set up with a cvs build and familiarise yourself with the
associated paraphernalia. Once you've got that I can point you at
various pieces of the source, but it will depend on which subsystem
you want to look at first.